definately the heater matrix... dont discount the headgasket etc but try the heater matrix. Fords systems are self bleeding and so there wont be an airlock and in there designs they put the heater matrix at the bottom of the system to prevent any air bubbles getting trapped. As a side effect of this the heater matrix gets clogged with shit.
I took mine out, took about an hour and it aint that hard, just fiddly. I flushed it through with a hose on on highish pressure.

As for the idling, im not sure, have you checked the connectors on top of your dizzy cap as this can effect the idle sometimes, mine was loose and earthing on the block, it made for the shittest running i have ever had lol!